Instead of incrementing it by one, I suggest to append current timestamp to the username which has more advantages compared to appending one. One advantage example is you can easily identify at what time the scripts failed just by seeing the id
Below is the code for appending timestamp for usernames
public string getUsername()
DateTime time = DateTime.Now;
string format = "dMMMdddhhmmyyyy"; // This you can change according to your need.
String username= "username_" + time.ToString(format) + DateTime.Now.Millisecond;
output : username_15dectue11172015468
Hope this information helps you.